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Thread: Peavey CS800S "Tempsistor" testing?

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    Member stoneattic's Avatar
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    Peavey CS800S "Tempsistor" testing?

    Hello all,

    I'm trying to repair an old Peavey CS800S power amp and the symptoms match what I have read to be a faulty "tempsister" (that's Peavey's name for it), which is one channel working and the other not, but showing the DDT led and the fans running at full speed.

    I emailed Peavey the serial number requesting schematics and they sent me the package below, which has schematics, parts lists, etc.:

    CS800S COMPLETE.zip

    The items in question are R206 and R207 on the Main Board schematic, which are called out as:

    235102AF 70240216 1K TEMPSISTOR HAND-INSERT MULTI-ROTATE FENWAL

    I pulled the tempsistors and measured them with my DMM. Initially the resistance is all over the place, from short to open, and then after awhile settles around 1K. Is this how they are supposed to measure? This is not the response my DMM usually has to normal resistors. The 1K makes me think they are okay, but I'm confused by the odd initial measurements. My understanding is that they are resistors that change resistance with temperature so I expect them to test like normal resistors.

    Any experience, suggestions, etc., would be appreciated! Thanks in advance!

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    Senior Member nevetslab's Avatar
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    Are the thermistors still in circuit, or now removed? I assume these are either beads, packaged somehow with short thin wire leads that are soldered into the PCB in some fashion. Thermistors are available in a variety of values, but in this application, they are no doubt NTC (negative temperature coefficient) type, meaning as they are heated, their resistance decreases. So, if you're holding one while trying to get a stable reading, your finger heat is causing them to change. That's what they do. Sit them on the bench, connect your DMM to the leads of each, and let them stablize to the temperature environment. Then, to verify they are working, you can press your finger to them, which should cause the reading to decrease, or even apply heat from your heat gun or soldering iron....just barely, as that is outside their working temperature range (heat sink temperatures range from room temp to around 80-90 deg C).

    If these thermistors have failed, you could place a resistance decade box between CR117 cathode and ground (they're using Chassis ground in this section of the schematic). I don't know if the fans run when the thermistors are at room temp or if they start up when the heat sink temp reaches 40 deg C. There is a small chart adjacent to C152 of the fan control circuit schematic, giving you several voltage levels that pertain to the temperature of the heat sink, and the resultant voltage applied to the van, so it definitely is a temp-variable speed control for the fans. You could use this to dial in 1k to start, and verify the fan changes speed as you decrease the resistance. Thermistors are readily available from vendors, both in bead form as well as some packaged.

    I had just ordered 10k NTC bead thermistors for a few bucks off ebay, and a package of 10 pcs of aluminum #6-32 x 1/4” male-female stud-standoffs, and dropped in a drop of freshly mixed 2-part thermal epoxy into the standoff holes, then dropped the bead probes into it and let them cure overnight. Used to replace a missing thermistor on a BGW GTC Power Amp heat sink, and controls the variable temp fan speed just like your Peavey CS800.

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    Member stoneattic's Avatar
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    Thank you for the reply nevetslab,

    I tested them out of circuit. They look kind of like a 1N4148 diode with clear plastic tubing over the exposed lead:
    CS800S tempsistor.jpg

    I assume the tubing is to prevent it grounding out on the heat sinks since they are right up against the heat sinks. It didn't occur to me when testing that my touching them would affect them, but now it seems so obvious.... once you pointed it out.

    Now I'm pretty sure they are fine but I'm thinking just to be sure while troubleshooting the amp I may replace them with regular 1K resistors. I don't believe that should cause a problem since I won't really be running the amp under any strain so fans speed shouldn't matter. If going to the resistors gets it working I will order new tempsistors from Peavey. If it doesn't, then I've eliminated a common cause.

    I was really hoping they were bad since that would be a simple fix. I'm not looking forward to figuring out what the problem is. It might be beyond my very limited skill set. I'm better with tube amps and pedals. Solid state amps are kind of voodoo to me.

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    Lifetime Member Enzo's Avatar
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    Well they are not regular resistors. They are temperature reacting resistors. If you measured them right after removing them, they could still be hot from the soldering process. Give them time to cool down and they measure right.

    Those thermistors are just one of several things that control the fans and lights. SO explore that whole mess of stuff down by Q109, Q110.

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    Supporting Member The Dude's Avatar
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    If you have one of the DDT lights on, I'm more apt to suspect a problem with that amplifier circuit. FWIW: My first check on a CS800 is to measure across the speaker connections with a DVM set to diode check (of course with power off). If the amp has shorted outputs or triac, the meter will usually tell you. You'll sometimes see caps charging, but the meter should eventually go to an open circuit reading. If it doesn't, you know where to look.

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    Member stoneattic's Avatar
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    I re-measured them this morning and they appear to be fine at 997 ohms. So I guess I'll reinstall them and start testing around Q109 and Q110 as Enzo suggested.

    The nice thing is I have a functioning channel to compare readings on. The bummer is the CS800S (2U) has them stacked so I can only measure one at a time powered up, where as the old CS800 (4U) had them side by side and was convenient to work on (but not to carry). I guess I can do an in circuit comparison un-powered as a first step? I know it won't tell me everything but might point out something really amiss?

    The Dude, does the test you suggested work for all solid state amps? As I said before I'm pretty new at this and it sounds like a great starting point.

    Thanks all!

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by stoneattic View Post
    Thank you for the reply nevetslab,

    I tested them out of circuit. They look kind of like a 1N4148 diode with clear plastic tubing over the exposed lead:
    CS800S tempsistor.jpg

    I assume the tubing is to prevent it grounding out on the heat sinks since they are right up against the heat sinks. It didn't occur to me when testing that my touching them would affect them, but now it seems so obvious.... once you pointed it out.

    Now I'm pretty sure they are fine but I'm thinking just to be sure while troubleshooting the amp I may replace them with regular 1K resistors. I don't believe that should cause a problem since I won't really be running the amp under any strain so fans speed shouldn't matter. If going to the resistors gets it working I will order new tempsistors from Peavey. If it doesn't, then I've eliminated a common cause.

    I was really hoping they were bad since that would be a simple fix. I'm not looking forward to figuring out what the problem is. It might be beyond my very limited skill set. I'm better with tube amps and pedals. Solid state amps are kind of voodoo to me.
    I haven't seen that form factor for a thermistor before. You're right, it does look a lot like a 1N4148, though thicker leads, and no cathode marking.

    Yeah, I can understand the feeling of being a bit overwhelmed by an amp like this, with SO MANY MORE PARTS than a tube amp to contend with, and all of them need to be operational so the current flow thru every stage works, and influences the next stage, all the way to finally tickling the loudspeaker properly. But, take heart...there are a lot of us here that will be glad to lend a hand and direct you, step by step as needed. Thankfully you have good documentation, and Peavey does a good job supporting their huge customer base.

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by stoneattic View Post
    ......The Dude, does the test you suggested work for all solid state amps?......
    Nope, not for all amps. Some amps have a relay on the output that closes when the amp is powered up. In that case, the output connections are basically disconnected with the amp off.

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    So I put the tempsistors back in tonight and noticed that R184 and R175 looked burned. I pulled them and tested them and R184 (should have been 47) read open and R175 (should have been 150) read shorted so I replaced them both. R184 is supposed to be flame proof, which I didn't have laying around so I put a regular carbon film in. I assume this is okay for testing/troubleshooting and if I get it fixed I will order the flame proof.

    I put the board back in and I am getting different results, unless I am mis-remembering (always a possibility).

    - With the "bad" board A completely unhooked channel B works fine and the fans turn slow as expected.
    - With the "bad" board A connected to the PS (and all other connections) I hear a relay click but no power anywhere, nothing comes on.
    - If I disconnect the PS connections to the "bad" board but leave the others connected channel B works fine, the fans run fast, and the DDT light is on on the "bad" channel A and that channel is dead (no "on" light, no sound)

    My recollection was that with the "bad" board A connected completely, including PS, channel B would work and the "bad" channel would show DDT and not come on. I'm wondering if I messed something up when I was doing my initial troubleshooting, which burnt the two resistors, which I don't remember being burnt before. Although if they cooked when I was under the hood I would have thought I would have noticed that.

    Nothing else looks visually damaged. My next step is to pull both boards and start taking resistance measurements, starting around the burnt resistors looks for differences. It looks like both resistors are right off the -75V PS connection so it makes me wonder if I connected the PS wrong at some point.

    Any thoughts (besides I may be an idiot), ideas, direction, etc.?

    As always, many thanks!

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    Senior Member nevetslab's Avatar
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    Most likely both R184 and R175 burned due to excess current being pulled by failed semiconductors. R175 is at the bottom of the voltage gain stage that includes the bias diodes to set up the output stage. The output stage, beginning with Q124/Q125 pre-drivers, Q126/Q127 drivers and Q128, Q129, Q130, Q131 & Q132, Q133, Q134 & Q135 output xstrs follows the two drive signals, and just provides the current to drive loudspeakers from that voltage gain stage & the two stages preceding it.

    I'm guessing you haven't become familiar with using your multimeter's diode test mode. Or, if it's an older multimeter not having one, you actually do. It's the 2k range in your ohmmeter. The diode test mode or the 2k ohmmeter range provides a 1mA constant current source, and on diode test modes, usually has a voltage source of around 3VDC. In the 2k Ohmmeter range, it's a 2VDC source. Good enough to test most semiconductors, though with GREEN LED's, you need more than 2V source to light it up (2.2VDC is typical for a GRN LED). But it's totally adequate to use for finding shorted semiconductor junctions on your NPN and PNP xstrs, signal diodes, rectifier diodes and zener diodes. Failure mode in solid state amps like your Peavey CS800S result in shorted transistors, shorted diodes, open junctions in transistors, shorted or open zener diodes and the like.

    This is what The Dude was referring to, looking between ground and the output terminal of the amp to see if there's a short. But, with relays connecting the amp to the speaker, usually amps like this would have DC Fault Protection, and keep the relay open. This amp has both relay connection with it's control logic, and a Triac circuit to impose a dead short across the output in the event one or more output transistors fail in a short.

    If testing xstrs and diodes is new to you with a multimeter, here are the basics. With the multimeter in diode test mode, red & black leads...the RED lead is the positive voltage source, the black lead is the return, and you are applying 1mA current thru the leads. Connecting the red lead to the Anode of the diode, and black lead to the cathode, you would typically read 0.665V or so....we can round it off to either 0.7V or 0.6V. That's a good semiconductor junction. With the leads reversed, you will measure open circuit. Testing an NPN transistor, connect the Red lead to the base, and black lead to the emitter, and you'll measure about the same 0.6 to 0.7VDC. Connecting the black lead to the collector would be the same. With a PNP transistor, you reverse the leads, with the black lead to the base, red lead to the emitter, then to the collector, and again, read about the same 0.6V to 0.7V. If measuring power transistors (like Q28 thru Q35), the reading will be lower, typically 0.51V to 0.56V, as there's more silicon in the semiconductor junctions.

    Zener diodes will measure a normal junction between anode and cathode (0.6 to 0.7V), while their real function is establishing a voltage reference, as found on CR120 1N751A, which is a 5.1V zener diode.

    Testing for shorts with everything connected (all soldered into the PCB, wired up, etc), you're looking for shorted semiconductor junctions on the diodes and transistors. I assume you know the symbols for NPN and PNP's. If not, the arrow pointing away from the base on the emitter is NPN, and if pointing toward the base, it's PNP.

    So, your first real task is to find and list all of the shorted semi's. There will be some, as R175 & R184 burning are solid clues. I'd begin with the voltage gain stage consisting of Q120 PNP and Q121 NPN, and the diodes between them CR127 thru CR130, as well as the diodes connected between the base and collector of Q120 and Q121 CR126 and CR131.

    If you find shorted pre-drivers, drivers and output xstrs, there are likely to be open emitter resistors that connect to the output buss that runs to the output relay thru an inductor & power resistor, ahead of the relay and triac circuit.

    I haven't looked at all of the circuits in this schematic to see if the DC fault sensing circuits can turn off the power supply to the 'failed' power amp. This should be enough to get you started.

    Also, since both amp channels are common to the +/- 75VDC power supply, your bad channel doesn't appear to be pulling it down and blowing fuses, so there's a good chance the damage is not in the output stages. But, you need to check them all. We can deal with all the rest of the front end circuits using op amps and such later. Look over Q114, Q115 and beyond at this point.

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    Member stoneattic's Avatar
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    Wow nevetslab, thanks so much for this!

    My DMM does have the diode test mode, but I've never used it for anything besides checking a diode out of circuit, and didn't realize I could test LEDs with it. I just used a 9v with a resistor to check them. That's going to save me a ton of time in the future.


    The more I think about it to more I think I caused additional damage (burning R175 and R184). I'm close to 100% sure they were not cooked when I first opened it up. The only thing I can think of is I must have reconnected the PS incorrectly at one point, so there is probably more than one "issue". The original and the one I likely caused.

    I will sit down this weekend and go over the entire board checking every diode and transistor. Hopefully they are readily available components, with the age of this guy I'm always a little worried about finding replacements.

    Thanks again!

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    Senior Member nevetslab's Avatar
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    When I was looking at the schematic, I'm seeing a lot of in-house semiconductor device numbers, as Peavey is one of many who have their own part numbers silk-screened onto the parts. I do have a list that came from Peavey that gives a cross-reference from their P/N's to the industry numbers, like they use in the front end of the discrete power amp. We can cross that bridge after you find what's ailing this. Since R175 burnt, also check R172, its' symmetrical counterpart off the positive supply rail. Both pass the same current, so it may also have been affected, as well as that positive rail series resistor R176.

    It looks like the Peavey in-house numbers begin with Q120 & Q121 upward. As for replacement parts that may be obsolete, I can help you out there with suitable parts that can be bought thru normal distrubuters like Mouser Electronics and such. Don't got the NTE route for replacements.

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    Member stoneattic's Avatar
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    I finally had a chance to start really digging in last night/this morning. Here's what I've found so far, and I believe it doesn't look good:

    I started with checking Q120 and Q121 and the diodes in between as nevetslab suggested using my DMM's diode test mode:

    "bad" board
    Q120 .463/.000
    Q121 .366/.458

    "good" board
    Q120 .464/.368
    Q121 .368/.454

    I found a cross-reference for Peavey transistors (I assuming the same one enevetslab has) and they appear to cross to MPS-U 60 and MPS-U 10 (both obsolete). When I look up the datasheets they are shown as a different form factor so I'm not sure which is the collector or emitter, but based on at least the Q120 measurements compared to the good board, they need to be replaced.

    CR126 .457/OL
    CR127 OL/OL
    CR128 OL/OL
    CR129 .519/OL
    CR130 .041/.041
    CR131 .462/OL

    The "good" board measured the same. CR127, 128, 129 where hard to find. They were in clear plastic tubes in the heat sinks. CR127 & 128 are dual diodes, which are new to me. I'm not sure if using the standard diode test setting is valid.

    I checked a bunch of other transistors and diodes as well (not all). Some appeared to check out good while some others confused me. I expected either something around .6 or .7 but some would have a reading in that range between the base and emitter or collector, but the other leg would read ~.1 (Q122, Q123). A couple read .000 (short) on one leg (Q112, Q124) so I assume they are bad. I still need to compare all the rest to the "good" board.

    Everything up to that point was tested in circuit. I decided to measure the big transistors (Q127 through Q135) since if they are failed they would likely be the most expensive to replace. Q128 though 135 appeared to be shorted in both directions. I compared what I was reading there with the "good" board and the "good" board transistors measured how I expected. When I measured CR140 and 141, since they appeared to "feed" those transistors, they tested as shorted on the "bad" board, but not the "good" board. I lifted one leg of each on the "bad" board and they then tested fine. I guessing that 129 through Q135 being shorted are creating a path around CR140 and 141?

    Do I need to lift a leg of everything that tested bad and recheck? Any chance the Q128 through 135 are NOT dead?

    So for the long babbling post but I'm trying to make sense of what I've seen so far.

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  14. #14
    Senior Member nevetslab's Avatar
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    I'm assuming your DMM semiconductor readings are in the form of "base-emitter/base-collector". I'm used to seeing somewhat higher junction readings, even when soldered in-circuit, and the probing order differs on Q120 & Q121 on the good board, but no matter. On the lower reading of those, you're probably seeing the effects of the resistors in parallel with the base-emitter junction, or the diode across the base-collector junction. Q120 is reading a shorted junction where you record .463/.000. I didn't realize these parts were TO-202 packages. Yes, those are obsolete. You can, though, replace them with MPSW-42 (NPN) and MPSW-92 (PNP), which is a 1W TO-92 plastic package, having the same pin-out mechanically. The schematic shows 3mA current running thru them, so with 73V across them, that's about 220mW. The smaller TO-92 1W package will run hotter than the TO-202 parts. To use a higher-wattage package, such as that of Q124/Q125, the pin-out is E-C-B left-to-right, so lead bending would be in order. MJE340 (NPN) and MJE350 (PNP) would be a suitable part, would run a bit cooler, it being a larger part.

    With CR127 & CR128 measuring the same on both boards, I'll take that as they're good on your bad board. Not sure why a dual-series-diode wouldn't measure around 1.20 to 1.30V, but, no matter. Those didoes CR127-CR130 are the thermally-coupled bias diodes, which track the temperature of the heat sink and keep up with the thermal change on the output stage.

    Q122 & Q123 are sensing the current thru the output stage emitter resistors, and as such, the base-emitter junction will read low, as you have 3 resistor networks of 1k + 0.33ohms in parallel, yielding about 334 ohms across the base-emitter junction of Q122/Q123.

    Yes, Q112 and Q124 would be bad with that reading. I forgot to mention that in checking xstrs, to check across the collector-emitter as well. Now, in circuit, that will often give confusing readings if you don't understand the circuit and what else is connected. When you do find shorted junctions, it's wise to check that C-E junction, particularly on the output stage. Q124 is a pre-driver, and if it had C-E short, that will make Q126 appear to have a shorted B-C short.

    Normally, on the output stage Q126 thru Q135, you should be reading good junctions between base and collector of all those. Base-emitter readings will appear low due to the emitter resistors all being common to the output buss, and give you very low readings.

    When you lifted CR140 and CR141 back-lash diodes across the output stages/output buss, did Q128 thru Q135 measure ok? Now, with the outputs in-circuit, the base-emitter junctions will read extremely low, but not shorted. If they read the same as your 'good board', then you're ok. Takes a little getting used to spotting a shorted output xstr that's one of several in parallel, but, there are differences found, where one or more that are truly bad will read lower than the others. You're measuring the base-emitter junction with 0.33 ohms in series with 22 ohms across the base-emitter junctions. The 22 ohms is the emitter resistor to output buss from the driver, which is in parallel with the outputs' base-emitter junctions, each with 0.33 ohms in series with their emitters to the same output buss.

    Were the backlash diodes CR140/CR141 ok?

    Another clue to suggest the output xstrs are ok, is that both amplifiers are powered from the same power supply. If one of the power amps' output stage had shorted outputs, it would pull high current, drag the power supplies down. Unless there are series fuses to each power supply of each power amp board. I didn't notice there being any. Quick way to tell is measure between the collector 'cases' of each amp to see if it's common (short). If not, then there may be fuses that are open on the bad amp.

    A further check, is, of course, to remove Q126 thru Q135 from the board, and check them out-of-circuit. When facing the bottom of the part, with the base & emitter resistors below the 'center-line', the terminal on the right is the base, terminal on the left is emitter, and collector, of course, is the case.

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    Member stoneattic's Avatar
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    I took the readings from the base to emitter and base to collector, but I didn’t differentiate, so I can’t tell which order I listed the values in. I didn’t look up which were NPN vs PNP but tried it one way and if it didn’t measure I flipped the DMM leads. I did not measure across the emitter-collector. I assumed all of them, except the big TO-3s, had the base as the middle lead and the collector and emitter as the outside leads.

    When I lifted CR140 and CR141 Q128 thru Q135 still test shorted. CR140 and CR141 tested good when lifted.

    I wouldn’t assume that any component failures happened in operation. I was not the first person to work on this. Someone else had been in there before and I very well could have caused some of the damage myself in my first attempts to figure out what went wrong.
    I’m thinking my next step is to pull the output xstrs and measure them out of circuit?

    Any hints on how to do that? On smaller xstrs and other components (resistors, diode, caps, etc.), I usually use my Hakko FX-888D set to 750°F and Soldapult. Occasionally on small items, or really tight areas like ICs, I will use solder wick. I haven’t tried yet, but the TO-3s on big heat sinks look like they require a lot more heat. I have a couple of old high wattage irons and guns that seem like they may be more appropriate for these.

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    Senior Member nevetslab's Avatar
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    What you could do is lift up the output xstrs' emitter resistors R187, 188, 195 thru 201, as well as R185, and the sense resistors R189 thru 194) at the emitter connections. That then leaves the base-emitter junctions open per xstr. The base-collector junctions are still all in parallel. So, if there is even one transistor shorted on the pos or neg side, all will read the same. Usually, when an output xstr fails, it fails with a C-E short as well as B-E and B-C short. And, unless the emitter resistor for that part failed open, it would pull the power supply down, and blow the mains fuse, unless there were power supply fuses per amp channel.

    You need to check the emitter resistors in the output stage first, as if you find any that are open, it's associated power xstr is likely bad.

    In how to do that (removing power xstrs), I had moved on from Soldapulit style solder extractors to Pace Desoldering irons with a full set of tips. Also a Weller Desoldering iron for their WTCP line. And, Solder Wick.

    Now, when CR140/CR141 were in circuit, and they tested short, but upon lifting, measured ok, that tells me you have Collector/Emitter shorts on both NPN and PNP sides of the output stage. That also suggests there are power supply fuses on the amp boards.

    In looking at the parts placement guide, super-imposed over the back foil pattern, I don't even see foil for the power supply rails bussing the collectors of all the power xstrs together. Nor have I found where CR140/CR141 are. I didn't see any fuses, but, I'm also not looking at the actual boards as you have.

    So, yes, you do need to find what has failed in the output stage, to yield the shorted readings you got on CR140/CR141.

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  17. #17
    Member stoneattic's Avatar
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    Thanks again nevetlab,

    I will lift the resistors you suggest and check. I'm pretty convinced all of the power xstrs are cooked, but maybe I'll get lucky. I'm thinking that at some point, either the other person that was in it, or me, hooked up the PS wires wrong or something. Maybe that's why he gave up and got rid of it. But of course I'd rather not put $50 worth of xstrs in if I don't have to.

    I've attached a copy of the PCB image with CR140 & CR141 circled.

    Click image for larger version. 

Name:	PCB with CR140 & CR141 circled.jpg 
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  18. #18
    Senior Member nevetslab's Avatar
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    Ah.....I must have had blinders on this morning. Also, from the 'blinders' effect, I see the power supply 'rails'. Each aluminum plate being used as the heat sink/heat spreader IS electrically connected to the TO-3 cases. No Insulator between the TO-3's and the heat sink. My bad.

    You might just have one pair of xstrs that failed. Without pulling up the emitter resistors, all of the base-emitter junctions will measure low, though not dead short, unless they all are. The base-collectors....I've already explained that, so you WILL have to remove the xstrs to find the culprits. I haven't looked up to see what Peavey installed. MJ15022's and MJ15023's are what most folks are replacing them with these days, using them in a complete set. They are a bit pricey....$5.56 ea on the MJ15022's is what I paid last month for a pair.

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  19. #19
    Member stoneattic's Avatar
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    I finally had a chance to pull the power xstrs yesterday evening and measure them. It was easier than expected. I just flipped the board over, soldapullt the leads and went back and forth trying to keep the solder molten and since the heat sink held them above the workbench, they just fell out. :

    Q128 (70473180)
    .488, .455, OL

    Q129 (70473180)
    000, 000, 000

    Q130 (70473180)
    .488, .443, OL

    Q131 (70473180)
    .488, .444, OL

    Q132 (70483180)
    .490, .461, OL

    Q133 (70483180)
    .490, .460 , OL

    Q134 (70483180)
    .127, .127, 000

    Q135 (70483180)
    .491, .463, OL

    All measurements shown in B-E, B-C, C-E order
    OL = open

    So it looks like Q129 and Q134 are toast. It seems odd to me that it wasn't a pair, but maybe I'm still thinking tube amp too much. Here are the pairs per the schematic, unless I'm reading something incorrectly (always a possibility). I've underlined the obviously bad ones.

    Q128 - Q132
    Q129 - Q133
    Q130 - Q134
    Q131 - Q135

    Am I safe just replacing those two or should I replace as pairs?

    My plan now is to retest the other xstrs and diode, just to be sure, then check resistors, etc. I'm not sure about how to go about testing the ICs though. After that, ordering all the parts.

    Any other hints or suggestions?

    Thanks!

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    Lifetime Member Enzo's Avatar
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    There are no pairs. There is a row on the plus side, and a row on the minus side. The fact they are drawn one above the other, or that they sit in the same positions in the two rows means nothing.

    Yes, those two look bad, so all that means is one of four from each side has shorted.

    It is typical that when one fails on the plus side, one will then fail on the negative side. Only a coincidence if the two seem "paired" up.

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  21. #21
    Senior Member nevetslab's Avatar
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    You've made good progress! I just looked up the Peavey P/N's 70473180 & 70483180 to find they're MJ15025G & MJ15024G TO-3 parts, readily available thru Mouser or Digi-Key Electronics (usually). Similar price to the MJ10522's I had mentioned. Also check R197 & R200 0.33 ohm 10W ceramic bathtub resistors. They being 10W parts they may be ok, since you had found CR140/CR141 measuring as a short until you lifted one end each. Now we're just checking to see if they measure the same value as the other's, since a lot of current was flowing thru them.

    Did you ever find a pair of fuses between the +/- High Voltage power supplies and the amp module where that connects? Shorted power xstrs on one of the two channels would normally pull sufficient current to blow the mains fuse, yet the amp works with just the good amp channel. I didn't see any circuit protection per channel to dis-connect the power amp from the amp's power supply. Not shown in the schematics.

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  22. #22
    Member stoneattic's Avatar
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    I finally had time to go over this one more time. I checked the resistors suggested (as well as all of the flame proof and big ceramic ones, I have plenty of 1/4W resistors so if I discovery any bad ones later that's not a problem) and didn't find any bad. I rechecked all of the xstsrs and diodes again. In all cases, except below, the "good" board and "bad" board read the same. Previously some looked shorted, but I hadn't compared it to the other board and/or I was measuring from the component side previously but switched to the trace side tonight.

    Q121 (I said Q120 in an earlier post, I think I had Q120 and Q121 backward when writing them down previously)
    I wanted to be sure it was not a nearby resistor causing the weird readings so I pulled Q121 and it is showing a short. The Peavey Cross Reference List says to replace it as a pair. I assume they mean along with Q120. As previously mentioned Peavey lists MPS-U10 and U60 as replacements, but are obsolete. I'm not sure what are safe replacements?

    There was another "weird" reading when comparing, Q119. In this case it actually seems like the "good" board might have a bad xstsr:
    "bad" board: .630/.630 reversed: OL/OL (OL across C-E)
    "good" board: .570/.590 reversed: OL/.920 (OL across C-E)
    Does it look like Q119 on the "good" board is on it's way out?

    At this point I think my shopping list is the following:
    qty 1 each of MJ15025G & MJ15024G
    qty 1 each of whatever will replace MPSU10 & MPSU60
    qty 1 47ohm FP 1/4W resistor (the one that burned)

    Should I pick up a replacement for Q119?

    I'm not sure about any capacitors. None are obviously cooked and I'm not sure how to really check them without pulling them.

    Am I missing anything else?

    Thanks as always!

    BTW, I don't see any fuses that would be specific to that channel. The GND, +75 and -75 don't have separate/different feeds to the main boards.

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    Last edited by stoneattic; 10-27-2018 at 03:11 AM.

  23. #23
    Supporting Member Jazz P Bass's Avatar
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    Q120 is a SDS7204 transistor.
    https://www.tubesandmore.com/product...7204-300v-10ma

    Q121 is a SDS7205.

    I would give Peavey Parts a call on that last one.

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  24. #24
    Senior Member nevetslab's Avatar
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    Suitable replacement parts for Q120 PNP would be a 2N5415 TO-5 metal can xstr, and for Q121, a 2N3440 NPN, also TO-5 metal can xstr. Same pin-out as the MPSU10 & MPSU60. As only 3 mA is flowing thru this stage, you could probably get away with the MPSW42 and MPSW92, which are 1W tall TO-92 xstrs. Not sure what Peavey has for substitutes. I don't think any semi mfgr is building TO-202 packages anymore. For Q119, a 2N5401 would suffice. The difference in readings on the two Q119's may have been from Q121 being bad, or removed from circuit. Both sets of readings do indicate good junctions.

    As you don't see any supply fuses, do an ohmmeter check between the two amp modules.....NPN T0-3 case on one to the TO-3 case of the other, as well as the PNP T0-3 cases. If you see near 0 ohm (wire resistance), then there aren't any fuses per module. I'm still puzzled how the supplies stayed up when you had shorted output xstrs. Anyone else curious about that?

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    Lifetime Member Enzo's Avatar
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    Nitpick: I call a 2N3440 a TO39 can, to me the TO5 is the smaller thing. I know not all agree with me.

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  26. #26
    Senior Member nevetslab's Avatar
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    Quote Originally Posted by Enzo View Post
    Nitpick: I call a 2N3440 a TO39 can, to me the TO5 is the smaller thing. I know not all agree with me.
    .....removing the egg from my face.....you're right! I had to go back to the data sheet on the 2N5415, and there it was...TO39.

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  27. #27
    Member stoneattic's Avatar
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    Thanks again!

    I added the 2N5415 and 2N3440 to my Mouser order. I'm not sure what else I should check, caps, etc., or am I at the point where I should just place the order and install everything and see where I am?

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  28. #28
    Senior Member nevetslab's Avatar
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    Quote Originally Posted by stoneattic View Post
    Thanks again!

    I added the 2N5415 and 2N3440 to my Mouser order. I'm not sure what else I should check, caps, etc., or am I at the point where I should just place the order and install everything and see where I am?
    As long as you have emitter resistors for Q120/Q121, and the 47 ohm series resistors that are used in the power supply lines for the 'front end stages', which do include Q120/Q121. The question about what else to order is always a tough one. Every time you find you need something else, there goes another shipping charge paid out to Mouser. I cringe every time I place an order, knowing the next amps on the bench will tell me I should have waited before ordering.

    I gather the associated emitter resistors for the two power xstrs WERE intact and not open. I think it's time to restore the circuit and see what else, if anything, is needed.

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  29. #29
    Member stoneattic's Avatar
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    I checked all of the resistors that were not 1/4w carbons, as I have plenty of those, and went ahead and placed the order. When I receive the parts I will install and report back.

    Thanks for all the help. Besides (hopefully) getting the amp back into service, I learned a good bit this time around.

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  30. #30
    Member stoneattic's Avatar
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    It took along time, but I finally had a chance to work on this again. I installed the new MJ15025 & MJ15024 power transistors and 2N5415 & 2N3440 voltage gain stage transistors, as well as reinstalling the other odds and ends I pulled/lifted during the troubleshooting last night and tested. Channel A is back and everything appears to be acting as it should!

    Thanks again for all the help!

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  31. #31
    Senior Member nevetslab's Avatar
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    Quote Originally Posted by stoneattic View Post
    It took along time, but I finally had a chance to work on this again. I installed the new MJ15025 & MJ15024 power transistors and 2N5415 & 2N3440 voltage gain stage transistors, as well as reinstalling the other odds and ends I pulled/lifted during the troubleshooting last night and tested. Channel A is back and everything appears to be acting as it should!

    Thanks again for all the help!
    Wow.....OUTSTANDING!! Glad to hear that.....feels really good, eh? My pleasure!

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  32. #32
    Member stoneattic's Avatar
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    Quote Originally Posted by nevetslab View Post
    Wow.....OUTSTANDING!! Glad to hear that.....feels really good, eh? My pleasure!
    Yes it does. I really thought I was in over my head on this one once the simple stuff didn't fix it. Well, actually I was, but you guys saved me. I learned a lot on this one.

    Thanks again.

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